1. Field of the Invention
The present invention relates to a multilayer ceramic capacitor and a method of manufacture thereof and, more particularly, to structure and a method of manufacture of a multilayer ceramic capacitor which realizes a low rate of change in electrostatic capacity, a small size, high capacity and low manufacturing cost.
2. Description of the Prior Art
A multilayer ceramic capacitor is used as a kind of capacitor and its ability to store an electric charge is given by electrostatic capacity. The rate of change in the electrostatic capacity resulting from temperature change is desirably as small as possible. In general, the capacitor having X7R characteristic is commonly used, in which the rate of change in electrostatic capacity is within .+-.15% in the temperature range of -55.degree. C. to +125.degree. C. However, if the Curie point of a ceramic acting as dielectric substance lies between -55.degree. C. to +125.degree. C., the electrostatic capacity thereof varies markedly depending on the change in ambient temperature. As a result, the capacitor function may be deteriorated.
For example, if a dielectric substance has the Curie point K1 of around -5.degree. C. as shown in a curve 81 of FIG. 1A, electrostatic capacity decreases more than 20% at 60.degree. C. or more. This is responsible for the malfunction of the capacitor.
In order to prevent such a problem, there is a way of varying the temperature characteristic by adding further chemical component to a ceramic material consisting mainly of barium titanate. This treatment serves to shift the Curie point to thereby make the electrostatic capacity stable. However, although this method is capable of decreasing the change in electrostatic capacity, it also reduces the dielectric constant down to the range of 2000 to 3000, leading to another problem that a large electrostatic capacity cannot be obtained.
Another way is to form a multilayer ceramic capacitor by laminating a plurality of ceramics having different Curie points. For instance, if a ceramic whose temperature characteristic is shown in a curve 81 of FIG. 1A is combined with another ceramic whose temperature characteristic is shown in a curve 82, the resulting ceramic capacitor has two Curie points and the rate of change in electrostatic capacity can be reduced on the basis of the logarithmic mixing law.
The conventional multilayer ceramic capacitor described above has the following problem. When manufacturing the above-mentioned capacitor, ceramic materials having different Curie points are first laminated and thereafter they are subjected to sintering as one united body. However, in this sintering step, ceramic materials are made to diffuse into one another. This is responsible for the problem that respective Curie points are nearly equalized. Due to this problem, it is impossible to construct a multilayer dielectric ceramic structure having a plurality of Curie points. As a result, the rate of change in electrostatic capacity cannot be reduced.
In order to prevent the above problem, another type of multilayer ceramic capacitor has been proposed in Japanese laid open publication No. SHO 64-64210(P). This multilayer ceramic capacitor is outlined in FIG. 1B. There are provided a plurality of first internal electrodes N1 and a plurality of second internal electrodes N2 in alternate order inside the capacitor. A first external electrode G1 and a second external electrode G2 are connected to side ends of respective internal electrodes. The capacitor shown in FIG. 1B employs two kinds of ceramics 85 and 86 as dielectric substance, each ceramic having different Curie point from one another.
An empty layer 87 is provided between ceramics 85 and 86, isolating two ceramics from one another. By the presence of the intervening empty layer 87, diffusion of ceramic materials in the sintering step can be hindered, thus preventing Curie points of respective ceramic materials from being equalized. This realizes a low rate of change in electrostatic capacity, with the dielectric constant maintained in high level.
However, the multilayer ceramic capacitor shown in FIG. 1B has the following problem. Although the empty layer 87 formed in the capacitor serves to relieve the diffusion of ceramic materials in the sintering step, it hinders the miniaturization of the capacitor due to its own space. In particular, it is necessary to laminate a large number of ceramic materials having different Curie points in order to make electrostatic capacity more stable. In such a case, empty layers should be formed between every adjoining ceramic materials. Consequently, the miniaturization of the capacitor is markedly hindered.
In addition, because the ceramic capacitor is formed by sintering at 1000.degree. C. or more, internal electrodes must be made of a heat resisting material such as palladium or the like. This results in the problem that the production cost of the capacitor is raised.